The invention relates to a method of recording holograms for use in optical scanners, comprising the steps of generating a reference beam by means of a reference source, generating an object beam by means of an object source, and illuminating a photosensitive surface with both the object beam and the reference beam. The invention further relates to an apparatus for implementing such a method, as well as to an optical scanner, more particularly a bar code scanner, comprising one or more holograms recorded according to such a method.
Holograms are widely used in active optical scanners as laser beam deflectors. The laser beam of the scanner is made to impinge upon a hologram and is subsequently deflected by the hologram towards a scanning plane, in which the symbol to be scanned will be located. As the laser beam is made to move relative to the hologram, the point of incidence of the laser beam travels across the surface of the hologram, producing a locus or path of the incident laser beam across this surface. This path of the point of incidence across the surface of the hologram causes the deflected beam to describe a certain path or scanning pattern across the scanning plane. As the beam deflection may depend on the particular point of incidence on the hologram, the scanning line or pattern generated by the laser beam on the scanning plane is influenced by the shape of the path of the beam across the hologram surface. That is, the locus of the laser beam determines, in conjunction with the deflection properties of the hologram, the scanning pattern across the scanning plane.
Many scanners in which holograms are used comprise a holographic disc, i.e. a disc containing several holographic facets and/or on which one or more holograms are mounted. Such a disc can e.g. be stationary, while the laser beam is rotating relative to the disc. The moving laser beam traverses each hologram of the disc in turn. As each hologram or holographic facet may produce a different scanning pattern, such as a single scanning line, each turn of the disc results in a number of scanning lines being produced. In order to obtain a multiple-directional scanning pattern, the holographic facets are in most scanners arranged in a circle, the laser beam sweeping the disc along a circular locus. This implies that the path of the beam across each hologram is curved, this path being an arc of a circle.
Conventionally, holograms are recorded using two sources, producing two beams: a reference beam and an object beam. A laser beam having the same wavelength as the reference beam and issuing from the same point will be deflected by the hologram in such a way, that the object beam is reconstructed. As the laser beam impinging upon a hologram during operation of an optical scanner, i.e. the reconstruction beam, generally issues from a single point, the reference beam employed for recording holograms for use in a scanner is preferably spherical. The object beam, however, is conventionally spherical or collimated. A collimated object beam will result in parallel deflected beams, each deflected beam originating from a point of incidence of the laser beam upon the hologram. Consequently, the arc-shaped path of the laser beam on the surface of the hologram normally results in a curved scanning line on the scanning plane and thus across the symbol to be scanned.
However, for many applications, e.g. for reading bar codes, it is desired to have substantially straight scanning lines. Curvature of the scanning pattern may result in the scanner being virtually unable to decode the scanned symbol. In the case of bar codes, a curved scanning line may not intersect all bars and consequently the code cannot be read.